How do I know when my chisels are sharp enough

A couple months ago I bought a 6 piece set of Marples Splitproof chisels second hand. They were in pretty poor shape when I got them. I spent a ton of time working on them, and now I can shave my arm hair with each of them, but is this really a good indicator that they are sharp enough for woodworking?

Shaving the hair on your arm is a decent indication, and quick to test while sharpening. Try paring end grain; do you get a shaving? Is the surface left smooth and shiny? If so, it’s probably sharp enough.

You can shave with a dull razor, so the arm hair thing really isn’t that indicative of sharpness.

Sharp = two planes (think geometry, not woodworking) coming together at a single intersection.

Ideally, the working end would be one molecule of steel thick. In reality, the best you can do is one grain of steel exposed. In practice, if you cannot see light reflected from the cutting edge, you are likely done. It will look like a black line while the rest of the bevel and back reflect light.

Pairing end grain is a decent enough test, use pine. It should happen nearly effortlessly and create shavings with some body, not dust.

Alternatively, you can hold a sheet of typing paper (copy paper) LOOSELY and push the chisel/blade down through the edge. Don’t pinch the paper to form a curve and make it more rigid. This is a cheat and you can push a much duller edge down through a rigid sheet than a “floppy” sheet.

Another simple test is the finger nail test. Carefully try to slide the sharp edge down the length of a thumbnail. It should stick and not slide. If it slides, it is dull. You can also draw your thumbnail across the edge and feel any micro nicks in the blade. Again, it should stick a bit but feel smooth as you draw across. Be careful with this test and don’t slice things that shouldn’t be sliced. Also, if fingernails on a chalkboard bother you, this test will also make your skin crawl.

Don’t spend all your time testing for sharpness. The wood you are working on will tell you if the tool is sharp enough. It should cut and you shouldn’t need to fight the tool, just guide it.

Also, invest in a x5 to x7 magnifying glass or eye loupe to inspect the lapping marks. Once you look at these a few times you will learn when you need to lap more (make the marks smaller) on any particular grit regardless of the presence of a burr. The higher the polish, the smoother the steel (in general) and so fewer longitudinal gouges in the steel which form the start of micro-fractures which eventually let a little flake come off and this is how your edge dulls.

I already reached the point of frustration, but I took a break and came back to it after I watched a bunch of youtube vids to make sure I wasn’t the problem. As it turned out, the chisels were just beat up, and needed a lot of work.

Just be aware that you can strop too much and dub the edge over. There is really no right or wrong. It is a balancing act. Too sharp and the edge doesn’t last. Too dull and it doesn’t cut as cleanly. There are times you want to change up for what you are doing. Cutting some nasty endgrain? Make it wicked sharp and stop and sharpen it again while you are cutting. Chopping out mortises? The same edge won’t last two chops. A soft strop will give you a longer lasting edge. A hard strop will give you a sharper one. That’s why barber’s stops have different belts (of course most people now days have never seen a barber’s strop)